Oils derived from plant materials, such as oil-seeds, cereal brans, fruits, beans, and nuts, are the source of raw material for many important commercial products. For example, such oils from such plant materials are extensively used in cooking, in cosmetics, as carriers for insecticides and fungicides, in lubricants, and in myriad other useful products. Consequently, much work has been done over the years in developing improved processes for extracting oil from such materials.
The most widely used process for removing oil from oil-bearing materials is solvent extraction. In solvent extraction, the oil-bearing material is treated with a suitable solvent, usually the lower carbon alkanes such a hexane, at elevated temperatures and pressures, to extract the oil from the oil-bearing material. The resulting solvent/oil mixture is then fractionated to separate the valuable oil from the solvent, which is recycled. Most solvent extraction processes in commercial use today employ hexane as the solvent. While hexane extraction is the most widely used today, there are also teachings in the art in which normally gaseous solvents are used at both supercritical and subcritical conditions.
One such teaching is found in U.S. Pat. No. 1,802,533 to Reid, wherein a normally gaseous solvent, preferably butane or isobutane, is liquefied by decreasing the temperature and/or increasing the pressure, then passing the solvent through a bed of the oil-bearing material in an extraction vessel. The solvent and extracted oil are then passed to a still where the solvent is separated from the oil. The extracted material must then be placed in another still where it is heated to remove solvent which remained entrained in the extracted material. There is no suggestion of obtaining a substantially solvent-free, dry extracted material without an additional treatment step after extraction.
Another extraction process is taught in U.S. Pat. No. 2,548,434 to Leaders wherein an oil-bearing material is introduced into the top of an extraction tower and passed counter-current to a liquefied normally gaseous solvent, such as propane, which is introduced at the bottom of the extraction tower. The tower is operated near critical conditions s that the solvent selectively rejects undesired color bodies, phosphatides, gums, etc. The resulting solvent/oil mixture can then be flashed to separate the solvent from the oil. In another embodiment, the solvent/oil mixture is first subjected to a liquid/liquid separation resulting in one fraction containing solvent and a less saturated fatty material, and another fraction containing solvent and a more saturated fatty material. The solvent is then flashed from both fractions. The extracted material remaining in the tower is drawn off and subjected to a vacuum flashing operation to remove entrained solvent.
Also, U.S. Pat. No. 4,331,695 to Zosel teaches a process for extracting fats and oils from oil-bearing animal and vegetable materials. The material is contacted with a solvent, such as propane, in the liquid phase and at a temperature below the critical temperature of the solvent to extract fat or oil from the material. The resulting solvent/oil mixture is treated to precipitate the extracted fat or oil from the solvent by heating the solvent to above the critical temperature of the solvent without taking up heat of vaporization. The extracted residue (shreds) is then treated to remove any entrained solvent, either by blowing it directly with steam, or by indirect heating followed by direct steaming.
Other references which teach solvent extraction of oil-bearing materials, with normally gaseous solvents, include U.S. Pat. No. 2,682,551 to Miller; and U.S. Pat. No. 2,560,935 to Dickinson. In each of these processes, the extracted material must be further processed to remove entrained solvent.
While prior art extraction methods, particularly hexane extraction, have met with various degrees of commercial success, there still remains a need in the art for an improved solvent extraction method which is more energy and cost efficient, and which is especially suitable for the processing of certain troublesome oil-bearing materials. One such troublesome material is rice bran, one of the most plentiful and nutritious food sources known to man, but which is greatly underutilized. This is primarily because immediately following the milling step, a lipolytic enzyme in the bran is activated which catalyzes the hydrolysis of the glyceryl esters of the free fatty acids (FFA) present in the lipids. This is measured by FFA increase, which is rapid at typical atmospheric storage conditions. These fatty acids leave the bran rancid in a matter of minutes after milling, and render it inedible to humans after several days of storage. Consequently, rice bran, as a source of oil and food, is underutilized, particularly in less developed countries. While food processors struggle to find ways to obtain a rice bran, and rice bran oil, free of these undesirable characteristics, more and more beneficial uses and nutritive values are being discovered for these products. For example, it has recently been reported that rice bran fiber is effective for lowering cholesterol in humans. As a result, a tremendous demand has been created for a process which can stabilize the rice bran after milling, or a process which will allow for the extraction of oil while at the same time stabilizing the oil and bran against further fatty acid formation.